In the field of silver halide photography, a photographic process capable of obtaining positive photographic images without forming negative images or without carrying out intermediate processing is called a direct positive photographic process, and photographic light-sensitive materials and photographic emulsions used for such a photographic process are called direct positive light-sensitive materials and direct positive photographic emulsions, respectively.
Although there are various kinds of direct positive photographic processes, a process which comprises exposing to light previously fogged silver halide grains in the presence of a desensitizer and thereafter developing and a process which comprises exposing to light silver halide emulsions having sensitizing specks in mainly the inner part of silver halide grains and thereafter developing in the presence of a nucleating agent are most available. The present invention relates to the latter process. Silver halide emulsions having sensitizing specks in mainly the inner part of silver halide grains wherein latent images are formed in mainly the inner part of grains are called inner latent image type silver halide emulsions, which are distinguished from silver halide grains wherein latent images are formed mainly on the surface of grains.
The process of directly obtaining positive images by surface development of inner latent image type silver halide photographic emulsions in the presence of a nucleating agent, and photographic emulsions and light-sensitive materials used for such a process are described in, for example, U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552 and 3,317,322, British Pat. Nos. 1,011,062, 1,151,363, 1,269,640 and 2,011,391, Japanese patent publication Nos. 29405/68 and 38164/74, and Japanese patent application (OPI) Nos. 16623/78, 137133/78, 37,732/79, 40629/79, 74536/79, 74729/79, 52055/80 and 90940/80, etc.
In the above described process of directly obtaining positive images, the nucleating agent may be added to a developing solution. However, a better reversal characteristic can be obtained when the nucleating agent is adsorbed on the surface of silver halide grains by adding the nucleating agent to photographic emulsion layers or other suitable layers in the light-sensitive materials.
As nucleating agents used in the above described process of directly obtaining positive images, hydrazines described in U.S. Pat. Nos. 2,563,785 and 2,588,982 and hydrazide and hydrazine compounds described in U.S. Pat. No. 3,227,552 have been known.
However, in the case of using these hydrazine compounds by adding the compounds to emulsion layers, it is necessary to use a considerably high concentration of the compounds (for example, about 2 g per mol of silver). Further, since the nucleating agent moves from the emulsion layers into the developing solution during development processing, the effective amount of the nucleating agent in the emulsions varies causing unevenness of maximum density (nonexposed part), and the nucleating effect in each emulsion layer becomes uneven in the case of multilayer color light-sensitive materials.
Further, it has been known that these nucleating agents generate nitrogen gas during the nucleating reaction. This gas gathers in the film to form bubbles which sometimes cause damage to photographic images.
As nucleating agents which avoid such faults, there are heterocyclic quaternary salt compounds described in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 4,094,683 and 4,115,122, British Pat. No. 1,283,835, and Japanese patent application (OPI) Nos. 3426/77 and 69613/77.
However, in silver halide photographic emulsions, sensitizing dyes are added generally for the purpose of spectral sensitization. In particular, in color photographic light-sensitive materials, since these materials require layers sensitive to green light and red light, respectively, in addition to an emulsion layer sensitive to blue light, emulsions for the green-sensitive layer and the red-sensitive layer contain sensitizing dyes for such purposes. When the nucleating agent is present in direct positive emulsions containing a sensitizing dye, competing adsorption of the sensitizing dye and the heterocyclic quaternary salt nucleating agent on silver halide grains occurs, and spectral sensitization is obstructed if the nucleating agent is added in an amount sufficient to form fog nuclei, while formation of fog nuclei is obstructed if the sensitizing dye is added in an amount sufficient to carry out spectral sensitization.
As a method of solving such a difficulty, U.S. Pat. No. 3,718,470 discloses a method using sensitizing dyes also having a nucleating substituent in the molecular structure.
However, in the method of giving a nucleating function and a spectral sensitization function to one molecule, there is a fault that the nucleating function becomes insufficient if used in an amount suitable for spectral sensitization, while the spectral sensitization becomes unsuitable if used in an amount sufficient for the nucleating function.
Further, the hydrazine compounds and the heterocyclic quaternary salt compounds have a common fault that the nucleating function of them has a large temperature dependence. Namely, the nucleating function deteriorates at a lower temperature, while sensitivity reduces at a higher temperature.
In order to solve such problems, it has been suggested to use thiourea attached acylphenylhydrazine compounds described in U.S. Pat. Nos. 4,030,925, 4,031,127, 4,139,387, 4,245,037, 4,255,511 and 4,276,364 and British Pat. No. 2,012,443, etc.
However, compounds described in the above described U.S. Patents are nearly insoluble in water and have very low solubility in organic solvents. Therefore, in order to add the above described compounds to hydrophilic colloid layers such as light-sensitive layers, etc., it is necessary to have a process which comprises dissolving once the above described compounds in a large amount of organic solvents and adding the resulting solution to a hydrophilic colloid solution. However, when a large amount of organic solvent is added to the hydrophilic colloid solution, hydrophilic substances such as gelatin, etc. contained in the solution easily cause separation or aggregation. If such a hydrophilic colloid solution is applied to a base, there is the possibility of causing uneven application and deterioration of the quality of light-sensitive materials because of precipitates or aggregates formed in the colloid layer.
In order to improve various properties of nucleating agents, there are other reports which disclose nucleating agents having a structure wherein various kinds of adsorbing groups are attached to an acylhydrazine skeleton as a nucleating site.
Namely, those using heterocyclic thioamide as an adsorbing group have been described in U.S. Pat. No. 4,080,207, and those using triazole derivatives have described in U.S. Pat. No. 4,278,748. However, because none of these compounds are sufficiently adsorbed on the surface of a silver halide, the amount of the compounds required is comparatively large or the nucleating function of the compounds has a tendency of having great dependence on processing temperature or preservation temperature of sensitive materials. Moreover, phenylacylhydrazine nucleating agents having a mercapto group containing heterocyclic group as an adsorbing group have been described in British Pat. No. 2,011,391B, wherein examples of the adsorbing group include groups of benzotriazole-2-thiols, 1-phenyl-5-mercaptotetrazoles and 1,2,3-benzotriazine-4-thiols. However, these nucleating agents have practical faults in that they not only have comparatively inferior preservation stability but also are difficult to synthesize because of their chemical structure.